fix: Validate when GSTIN field is empty (backport #2643)

[mergify]

Issue Reference: https://support.frappe.io/helpdesk/tickets/22783

_{Huly®: IC-2767}

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This is an automatic backport of pull request #2643 done by Mergify.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 59.85%. Comparing base (53e0fcb) to head (3bdd635).
Report is 4 commits behind head on version-15-hotfix.

Files with missing lines	Patch %	Lines
.../gst_india/overrides/subcontracting_transaction.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           version-15-hotfix    #2644   +/-   ##
==================================================
  Coverage              59.85%   59.85%           
==================================================
  Files                    120      120           
  Lines                  11619    11619           
==================================================
  Hits                    6955     6955           
  Misses                  4664     4664           

Files with missing lines	Coverage Δ
.../gst_india/overrides/subcontracting_transaction.py	29.23% <0.00%> (ø)

[india-compliance-bot]

🎉 This PR is included in version 15.13.4 🎉

The release is available on GitHub release

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